Back then home computers were the Apple II, the commodore PET and the TRS80. IBM just introduced the PC. While we can upgrade and replace our computers fairly easily, it is a lot harder for NASA. They have to make sure any new instruments are compatible with everything else and with no chance of failure.
You will find the same situation in older nuclear power plants, worse in older water and waste treatment plants (a lot less money than NASA for upgrades). Things are often identical to when they were built 50 years ago. I still remember a TV station checking out the subway system for Y2K readiness. The big board was still using incandescent pilot lights to indicate where the trains were!